1. Field of the Invention
The present invention relates to flame-retardant electrical cables, in particular for low-voltage power transmission or for telecommunications. Also, the invention relates to flame-retardant compositions, suitable for producing said cables.
2. Description of the Related Art
Flame-retardant cables are generally produced by extruding over the core of the cable a flame-retardant coating consisting of a polymer composition which has been given flame-retardant properties by the addition of a suitable additive. Polyolefin-based compositions based, for example, on polyethylene or ethylene/vinyl acetate copolymers, containing an organic halide combined with antimony trioxide as flame-retardant additive can, for example, be used for this purpose. However, halogenated flame-retardant additives have many drawbacks, since they partially decompose during processing of the polymer, giving rise to halogenated fumes that are toxic and corrode the metal parts of the polymer-processing equipment. In addition, when they are placed directly in a flame, their combustion gives rise to very large amounts of fumes containing toxic substances. Similar drawbacks are encountered when polyvinyl chloride (PVC) with added antimony trioxide is used as base polymer.
In recent years, use has been made of halogen-free compositions in the production of flame-retardant cables, in which a polymer base, generally made from at least one polyolefin, is mixed with inorganic flame-retardant fillers, generally hydroxides, hydrated oxides or hydrated salts of metals, in particular of aluminium or magnesium, such as magnesium hydroxide or alumina trihydrate, or mixtures thereof (see, for example, U.S. Pat. Nos. 4,145,404, 4,673,620, EP 328,051 and EP 530,940).
However, the use of inorganic flame-retardant fillers, such as magnesium hydroxide or alumina trihydrate, does have certain drawbacks. Particularly, to impart effective flame-retardant properties, large amounts of the inorganic filler should be added to the polymer material, e.g. for magnesium hydroxide generally about 120-250 parts by weight relative to 100 parts by weight of the polymer material. Such high levels of filler lead to a reduction in processability and in mechanical and elastic properties of the resulting mixture, in particular as regards impact resistance, elongation and stress at break.
In U.S. Pat. No. 5,707,732 an electrical or telecommunications cable is disclosed, which is coated with a flame-retardant composition comprising 100 parts by weight of a resin mixture and from 5 to 250 parts by weight of a flame-retardant filler. The filler is either magnesium hydroxide or aluminium trihydrate, while the resin mixture consists of: (i) a polyethylene made using a metallocene single-site catalyst system and having an Mw/Mn ratio not greater than about 3; (ii) a polyethylene made using a transition metal catalyst other than a metallocene single-site catalyst system and having an Mw/Mn ratio greater than about 4; and optionally (iii) a copolymer of ethylene and an unsaturated ester or a very low density polyethylene having a density not greater than 0.915 g/cm3; wherein resins (i) or (ii) are modified with an unsaturated aliphatic bi-acid anhydride through grafting or copolymerization.
International Patent Application WO 99/05688 discloses low-smoke self-extinguishing cables, which are coated with a flame-retardant composition comprising natural magnesium hydroxide as flame-retardant filler and, as polymeric components: (a) a crystalline propylene homopolymer or copolymer; and (b) a copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with a diene, said copolymer being characterized by a composition distribution index greater than 45%, said index being defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin. The copolymers (b) can be obtained by copolymerization of ethylene with at least one alpha-olefin, and optionally with a diene, in the presence of a single-site catalyst, particularly a metallocene catalyst or a constrained geometry catalyst. These compositions allow producing cables which combine a high flame-resistance with good mechanical properties. Particularly, the ethylene/alpha-olefin copolymer as defined above improves incorporation and dispersion of the mineral filler into the polymer matrix, while the presence of crystalline propylene homopolymers or copolymers enhances thermocompression resistance of the cable.
International Patent Application WO 00/19452 discloses low-smoke self-extinguishing cables, which are coated with a flame-retardant composition comprising: (a) an ethylene homopolymer or copolymer having a density of from 0.905 to 0.970 g/cm3, and being selected from: ethylene homopolymers; copolymers of ethylene with an alpha-olefin; copolymers of ethylene with an ethylenically unsaturated ester; or mixtures thereof; (b) a copolymer of ethylene with an alpha-olefin, and optionally with a diene, said copolymer (b) having a density of from 0.860 to 0.904 g/cm3, and being characterized by a composition distribution index greater than 45%; (c) natural magnesium hydroxide in an amount such as to impart flame-retardant properties; wherein at least one of the polymeric components (a) and (b) contains hydrolysable organic silane groups grafted onto the polymer chain.
U.S. Pat. No. 6,384,143 relates to a rubber composition which contains a thermoplastic partially or completely crosslinked product comprising 1-99 parts by weight of an ethylene-α-olefin copolymer (A) containing an ethylene unit and an α-olefin unit of 3-20 carbon atoms and prepared using a metallocene catalyst and 1-99 parts by weight of an olefin resin (B)(the total amount of (A) and (B) being 100 parts by weight), the crosslinking degree of the ethylene-α-olefin copolymer (A) being 50% or higher, and 1-500 parts by weight of a thermoplastic elastomer (C) added later to the crosslinked product. The olefin resin (B) can be an ethylene resin or a propylene resin. The propylene resins usable most suitably in the above invention include, for example, isotactic polypropylene homopolymers and isotactic copolymer resins. The thermoplastic elastomer (C) may be selected from, inter alia, ethylene-propylene rubbers, ethylene-propylene-diene monomer terpolymer rubbers (EPDM) and ethylene-octene-copolymer rubbers. The composition can be produced through a method which comprises subjecting the copolymer rubber (A) and the olefin resin (B) to dynamic crosslinking, then adding the thermoplastic elastomer (C) at the latter stage of the same extruder, melt kneading them, and then removing the composition from the extruder. The composition may contain, inter alia, inorganic fillers and flame retardant additives. The compositions can be used for a wide variety of uses, including, inter alia, cables.